First to use in commerce, not first to register is the US rule. Registration has real benefits, but even if it looks simple online, I advise you to get legal counsel and do it right, not just fast. One lives with the consequences of imperfect registraition for the life of the business. Answering the other part of your question, you declare your dates of first use, without the need of proof at the fiing stage, but you may be required to provide proof of first use later so make sure you keep/kept your records well.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Before applying to federally register trademark rights, the alleged rights owner must first declare under penalty of perjury: "That the applicant believes it is the owner of the mark; that the mark is in use in commerce; that to the best of the declarant's knowledge and belief, no other person has the right to use the mark in commerce, either in the identical form or in such near resemblance as to be likely, when applied to the goods or services of the other person, to cause confusion or mistake, or to deceive; that the specimen shows the mark as used on or in connection with the goods or services; and that the facts set forth in the application are true." 37 CFR 2.33(b)(1).
If the rights owner cannot truthfully take that oath it cannot lawfully apply to federally register its trademark rights [though there is a "concurrent use" application exception]. If it does anyway -- and registers the same mark that it knows someone else is entitled to federally register to brand the same goods -- then that other person can sue the registrant for damages. See 15 USC 1120.
So ... if a trademark rights owner knows that another is using the same or a confusingly similar mark in the marketplace to brand the same or related goods, then before applying to register its rights the rights owner MUST engage in due diligence to determine if that other user's rights were created first [that is, are they "senior"] or otherwise "stronger." That due diligence must be done by a trademark attorney. Good luck.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
I agree with my colleagues, and would only add that you don't "get the exclusive right to use" the mark in a vacuum. A registration will provide you with a presumption of such exclusivity in connection with the goods or services specified in the certificate of registration. This presumption is rebuttable, and, as my colleague states, a third party with superior (prior) common law rights could challenge your registration, as well as your use of an identical or confusingly similar mark for identical or related/complementary goods or services. Additionally, please bear in mind that the USPTO will not compare your applied-for mark with common law marks (those not registered or pending with the USPTO) in a likelihood of confusion analysis. The USPTO only examines trademark applications vis-a-vis other applications and registrations. That is why a comprehensive search and clearance opinion from qualified trademark counsel is a very important step in the process. Good luck.
Notice: This answer is for educational and informational purposes only, is not a reflection on or a representation of any views or opinions held by my employer, and should not be construed as legal advice. Nothing herein is meant to create an attorney-client relationship.
This is a tricky situation. Rights go to the first user of a trademark. However, those rights only extend geographically as far as the trademark has been used, or is known to the public. For very local uses of a trademark (e.g. a mom and pop store) this can be a significant issue. Thus, there can be more than one rightful owner of a trademark in different parts of the country. All of this so far has assumed no trademark registrations (as in your case).
When you go to register your trademark, you must file a sworn statement to the effect that you are not aware of any other rightful users of the trademark. This opens up a huge can of worms and a whole law review article can be written on the subject but to keep it short, THE LESS YOU KNOW THE BETTER. Simply being aware of someone else USING the mark is not enough to create a disclosure obligation on your part. So go ahead and file for the trademark! The USPTO will NOT refuse your trademark based on the other person's use because the examining attorneys there do not have access to such information. They will only cite prior trademark applications or registrations. So in a sense it is a race to the USPTO. Let's assume that you win the race. Here are some scenarios for what can happen (the assumption is that neither party knew of the other until some time after they both had ben using the trademarks).
1) the other user used her mark before you did - that other user can successfully oppose your application and possibly even prevent you from using your trademark (that depends on whether there is a geographical overlap OR even whether there is not but the prior user has been steadily expanding since her first use and would inevitably have gotten to your area....it's an equitable determination to be made by a court).
2) you are the first user - you can prevent the other person from using her trademark (the same conditions mentioned above hold true here).
3) the other person used her trademark first BUT because the conditions mentioned in scenario 1 are not met, you have a right to use your trademark in the geographical area you have "claimed" UP TO THE POINT IN TIME THAT THE OTHER USER FILES A TRADEMARK APPLICATION, IF EVER. In this case you are considered a "good faith junior user" and will be able to continue using your trademark notwithstanding the prior geographically remote user.
This is complicated for sure. It can get even more complicated with different facts but hopefully this answers the heart of your question.